The present invention relates to a time domain multiplexer, and more particularly to the arrangement of a multiplexer for use in a system in which a plurality of digital channel signals including channel signals of unequal bit rates are subjected to time domain multiplexing and then are transmitted.
One of the advantages of digital signal transmission is that a plurality of information (channel signals) can be transmitted through a single transmission line in a time-domain-multiplexed fashion and that economy can be attained by sharing the transmission line and circuit devices. In order to perform time domain multiplexing, however, the bit rates of the respective channel signals must have a fixed relationship. In particular, in case of multiplexing a plurality of channel signals whose bit rates are extremely different, for example, differing by several times to several tens of times, a multiplexing system of the so-called hierarchic structure is needed. As will be explained in detail later, hierarchic multiplexing is performed in such a way that signals of a plurality of channels having equal bit rates are subjected to time domain multiplexing, the multiplexed signal is multiplexed with signals of another channel, or another time-domain-multiplexed signal having a bit rate equal to the bit rate of the former signal, into a time-domain-multiplexed signal having a still higher rate (being, for example, double the bit rate), and such time-domain-multiplexing operations are then hierarchically combined.
In case of carrying out the multiplexing hierarchy method as described above, a time domain multiplexer must be designed for adaptation to a particular transmission system, i.e., to take into consideration the number and bit rates of the channel signals to be multiplexed. However, it is uneconomical and forms a cause for high cost that a special time domain multiplexer must be designed and fabricated for every transmission system. When the apparatus of this type is constructed of a large-scale integrated circuit (LSI) with the intention of providing the time domain multiplexer as a general-purpose device which is applicable to different multiplexing transmission systems, exclusive circuits corresponding to various bit rates must be additionally provided so that the various bit rates can be accommodated. This results in a bulky apparatus or a low transmission efficiency. Conversely, when an exclusive multiplexing apparatus is fabricated for a specific multiplexing transmission system, the transmission efficiency is enhanced, but the general-purpose applicability thereof is lost. In case the degree of multiplexing of a previously-used channel has changed or the bit rate has changed, it is required on occasion to alter the connections or to change the exclusive multiplexing apparatus to a significant extent.